There are many known power assist steering systems. Some provide assist by using hydraulic power and others by using electric power. Examples of electric power assist systems are shown in U.S. Pat. Nos. 3,983,953 and 4,415,054.
In U.S. Pat. No. 3,983,953, the electric motor is coupled to the input steering shaft and energized in response to the torque required in turning of the manually operated hand wheel. When the hand wheel is rotated, a torque sensitive switch actuates the electric motor. The electric motor applies a drive force to the pinion and thereby assists the driver with the steering maneuver.
In particular, the '953 patent utilizes a steering shaft having two coaxial shaft sections rotatable with respect to each other. A potentiometer is connected to one of the shaft sections and the wiper arm for the potentiometer is coupled to the other shaft section through a gear arrangement. The resistance across the potentiometer is indicative of the amount of relative rotation between the two coaxial shaft sections. Two spaced apart direction contacts are provided coupled to one shaft section. A contact arm is located between the two direction contacts and is coupled to the other shaft section. When sufficient rotation occurs between the two coaxial sections, contact is made between the contact arm and one of the direction contacts. The amount of relative rotation is limited. A pin is connected to one shaft section and includes two wings which extend into a slotted sleeve arrangement connected to the other shaft section. Another embodiment of the '953 patent has a reversing switch that directly applies a bias voltage to the electric assist motor after a predetermined amount of relative rotation occurs between the two shaft sections
U.S. Pat. No. 4,415,054, utilizes an electric assist motor having a rotatable armature encircling a force transmitting member. Rotation of the electric assist motor armature causes linear movement of the force transmitting member. The electric motor is energized in response to rotation of the steering wheel.
In particular, the '054 patent uses a steering column having two coaxial shaft sections rotatable with respect to each other. A Hall effect sensing device is located between the two steering shaft sections. A torsion bar mechanically couples the two steering shaft sections. The torsion bar is deflected in response to torque created by rotation of the hand wheel and the steered wheel resistance to turning. The amount of deflection can be used to determine the amount of assist that the system will give to a steering maneuver. An electric signal is produced by the Hall effect sensing device which is indicative of (a) the amount of relative rotation between the two shaft sections and (b) the direction of relative rotation. This signal from the Hall effect sensing device is used to control the electric assist motor.
Previous electric assist steering systems have not been concerned with the effects of electrical failures that may occur within the control system. In such systems, it is possible that an undesired electrical drive potential can be outputted to the electric assist motor upon a control system failure that would turn the vehicle when the operator does not desire nor expect such turning to occur. One example of such a system uses a Hall effect device and a processing circuit that outputs a voltage of 6 V.D.C. to keep the vehicle wheels in a known steered condition. When steering is desired, the control system generates a D.C. voltage either greater or less than 6 V.D.C. depending on the desired steering direction. If an electrical failure occurs, a signal far greater or less than the desired 6 V.D.C. can be generated causing the vehicle to take a hard turn in an undesired and unexpected manner. Also in such a system, spurious noise signals could cause actuation of the electric motor during driving which would, in turn, cause undesired and unexpected steering of the vehicle wheels.
Previous electric assist steering systems have utilized complex circuitry that (i) sensed the direction of the steering maneuver, (ii) sensed the amount of torque applied to the vehicle hand wheel, (iii) determined the amount of electric assist to provide and (iv) connected the determined drive signal to the electric motor in a manner to drive the motor in a direction responsive to the sensed direction of the steering maneuver. To improve reliability, it is desirable to reduce the amount of electronics required to accomplish the desired control of the electric assist motor.
Previous electric assist steering systems have had a problem with electric arcing across the switch contacts that connect an electric drive potential to the electric assist motor. Arcing occurs when an electric potential is present across the switch contacts prior to contact closure or present during contact opening. Continuous arcing causes premature wear and subsequent failure of the contacts. It is important that the reliability of an electric assist steering system be at least equivalent to the reliability of a hydraulic, power assist system. However, present control systems do not prevent arcing across switch contacts and, therefore, have a shorter contact life and lower operating reliability than is desired.
Previous electric assist steering systems typically would sense the direction of a steering maneuver using a sensor, such as a Hall effect device or electric contacts closed by the mechanical action of the steering maneuver. These sensed direction signals would be fed to an electronic control unit which would control application of an electric drive signal to the motor in response to the sensed signals. To improve system reliability, it is desirable to utilize the mechanical action of the steering maneuver to directly connect an electric drive potential to the electric assist motor. One proposed system divides the steering shaft into coaxial sections coupled by a torsion rod. Electric contacts are mounted on the relative rotation stops of the coaxial shaft sections. Upon relative rotation of the two shaft sections, electrical contact is made directly from the vehicle battery to the electric motor when the stops are reached. Such a system (i) does not provide for switch closure upon a small degree of rotation, (ii) does not permit continued relative rotation between the two shaft sections after contact closure occurs, and (iii) permits electrical arcing across the switch contacts.
An object of the present invention is to provide a control apparatus for an electric power assist steering system that will not output an effective drive potential to an electric motor of the electric power assist system if a system failure occurs when the operator does not desire to make a steering maneuver. If system failure occurs, it is desirable to steer the wheels only when the operator wishes to make a steering maneuver and that the failure only affect the degree of steering assist and not cause uncontrolled steering action.
Another object of the present invention is to provide a control apparatus including a simple drive circuit that outputs an effective drive potential to an electric motor of an electric power assist steering system. It is also desirable to direct the effective drive potential to the electric assist motor by the mechanical action of the relative rotation between the input shaft and the pinion shaft. This greatly reduces the amount of electronics involved.
Still another object of the present invention is to provide a control apparatus that does not permit an electrical drive circuit to output an electrical drive potential until electrical connections are made between the electric assist motor and the electrical drive circuit to prevent arcing across electrical contacts of connection switches.
Yet another object of the present invention is to provide a control apparatus for an electric assist steering system that (i) directly connects a variable drive voltage to an electric motor of the electric assist steering system upon a small degree of relative rotation between two coaxial steering shaft sections and (ii) permits continued relative rotation between the two steering shaft sections after the variable drive potential is connected to the electric motor.